The inv(16) is one of the most frequent chromosomal rearrangements associated with acute myeloid leukemia. In biological and in vitro assays, the inv(16) fusion protein blocks transcription of AML-1/CBFbeta- regulated genes. In preliminary studies, the applicant demonstrated that the inv(16) fusion protein cooperates with AML-1 to repress transcription. This cooperativity requires the ability of the translocation fusion protein to bind to AML-1. Mutational analysis and cell fractionation experiments indicate that the inv(16) fusion protein acts in the nucleus and that repression occurs when the complex is bound to DNA. He also found that the inv(16) fusion protein binds to AML-1 when it is associated with the mSin3A corepressor. An AML-1 mutant that fails to bind mSin3A was impaired in cooperative repression, suggesting that the inv(16) fusion protein acts through mSin3 and possibly other corepressors. Finally, he demonstrated that the C-terminal portion of the inv(16) fusion protein contains a repression domain. He hypothesizes that the fusion of CBFbeta with the smooth muscle myosin heavy chain gene MYH11 creates a transcriptional corepressor for AML-1B that promotes leukemogenesis by repressing the p21Waf1/Cip1 cyclin dependent kinase inhibitor and the p14ARF and neurofibromin tumor suppressor genes. He will define the molecular mechanism of transcriptional repression by the inv(16) fusion protein, and determine if p21Waf1/Cip1, p14ARF and Nf1 are direct targets for regulation by the inv(16) fusion protein. In addition, he will determine the contribution of these putative inv(16)-regulated genes to leukemogenesis by creating mice lacking p21Waf1/Cip1, p14ARF and Nf1. By determining the mechanism of inv(16)-mediated repression and identifying critical target genes for regulation, he will begin to elucidate the molecular mechanism of inv(16)-mediated leukemogenesis.